η1-Vinylidene formation from internal alkynones by C-C bond migration

Article abstract of DOI:10.1002/ejic.200700588

The [μ²-N₂(CpRu(PPh₃)₂) ₂]²⁺ (1²⁺) dication is a source of the formal 16-electron [CpRu(PPh₃)₂]⁺ fragment, and it reacts with internal alkynes such as 1,3-diphenylpropynone, 4-phenyl-3-butyn-2- one, and 3-cyclopropyl-1-phenylpropynone to yield η¹-vinylidene complexes 2a⁺-2c⁺ by C-C bond activation. In the case of 1,3-diphenylpropynone, the η¹-ketone complex [CpRu(PPh ₃)₂{PhC(η¹-O)C≡CPh}]⁺ (3a⁺) was isolated as its PF₆^- salt, and it is apparently an intermediate in the formation of the vinylidene complex. Complexes 2a⁺-2c⁺ were characterized as their BAr^F ₄^- [ArF = 3,5-bis(trifluoromethyl)phenyl] salts by NMR and IR spectroscopic methods, ESI-MS, and EA. The single crystal X-ray diffraction structure of 2aBAr^F₄ is presented. These reactions represent the first examples where an internal alkyne undergoes C-C bond migration to form an η¹-vinylidene complex. Wiley-VCH Verlag GmbH & Co. KGaA, 2007.

Full text of DOI:10.1002/ejic.200700588

SHORT COMMUNICATION
DOI: 10.1002/ejic.200700588
η1-Vinylidene Formation from Internal Alkynones by C–C Bond Migration
Michael J. Shaw,*[a] Steven W. Bryant,[a] and Nigam Rath[b]
Keywords: Ruthenium / Alkynes / Isomerization / Vinylidene bond activation / Unsaturated complexes
–
The [µ2-N2{CpRu(PPh3)2}2]2+ (12+) dication is a source of the
formal 16-electron [CpRu(PPh3)2]+ fragment, and it reacts
with internal alkynes such as 1,3-diphenylpropynone, 4-
phenyl-3-butyn-2-one, and 3-cyclopropyl-1-phenylpropy-
none to yield η1-vinylidene complexes 2a+–2c+ by C–C bond
activation. In the case of 1,3-diphenylpropynone, the η1-
ketone complex [CpRu(PPh3)2{PhC(η1-O)CϵCPh}]+ (3a+) was
Complexes 2a+–2c+ were characterized as their BArF [ArF
4
= 3,5-bis(trifluoromethyl)phenyl] salts by NMR and IR spec-
troscopic methods, ESI-MS, and EA. The single crystal X-ray
diffraction structure of 2aBArF4 is presented. These reactions
represent the first examples where an internal alkyne un-
dergoes C–C bond migration to form an η1-vinylidene com-
plex.
–
isolated as its PF6 salt, and it is apparently an intermediate
in the formation of the vinylidene complex.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2007)
“[CpRu(PPh3)2]PF6” can be prepared in situ from
[CpRu(PPh3)2Cl] and AgPF6 in CH2Cl2, but the actual
metal species has proved difficult to isolate and character-
ize, a circumstance previously described for several related
unsaturated metal complexes.[10] A tractable material that
serves as a synthon for the “[CpRu(PPh3)2]+” unit is [µ2-
N2{CpRu(PPh3)2}2](BArF4)2 [1(BArF4)2, ArF = 3,5-bis(tri-
fluoromethyl)phenyl]. The addition of 1,3-diphenylpropy-
none, 4-phenyl-3-butyn-2-one, or 3-cyclopropyl-1-phenyl-
propynone to CH2Cl2 or CDCl3 solutions of 1(BArF4)2 im-
mediately and cleanly yield the η1-vinylidene complexes
2aBArF4–2cBArF4, respectively (Scheme 1).
Introduction
Terminal vinylidene complexes have an established role
in the catalytic and stoichiometric transformation of al-
kynes into elaborated structures.[1] The isomerization of al-
kyne ligands to vinylidenes has been examined in detail by
experimental[2] and theoretical techniques[3] and this reac-
tion continues to generate interest.[4] A net 1,2-H atom shift
is common,[5] but the migration of other functional groups
is limited to –SiMe3, –SR, and –I,[1,6] which means that fur-
ther reactions are necessary to produce disubstituted vinyl-
idenes. In only one case has C–C bond activation been ob-
served in an alkyne-to-vinylidene conversion,[7] although C–
C bond migration in vinylidene-to-alkyne isomerization has
been reported.[8] This communication details the formation
of terminal vinylidene complexes through C–C bond acti-
vation of acetylenic ketones by electrophilic cations derived
from [CpRu(PPh3)2Cl], a well-known starting material for
the formation of vinylidene ligands from terminal alkynes.[9]
Results and Discussion
Scheme 1. Formation of vinylidene complexes from acetylenic
ketones.
Halide abstraction of Cl– by silver salts from [CpRu-
(L)2Cl] (L = phosphane) results in the formation of unsatu-
rated cationic complexes that absorb N2 even from high-
The single-crystal X-ray diffraction study (Figure 1)[11] of
2aBArF4 reveals the vinylidene cation 2a+ and clearly shows
that an unexpected C–C bond migration has taken place.
This rearrangement represents an unprecedented rearrange-
ment for an internal alkyne in a monometallic compound.
The structure of 2a+ is comparable to similar structures
of [CpRu(L)2(vinylidene)]+ cations (L = phosphane) with a
Ru=C bond length of 1.818(4) Å, a C=C bond length of
1.335(5) Å, and P–Ru–C bond angles of 94.65(11)° and
98.30(3)°.[12] This structure clearly establishes that C–C
bond migration occurs, which transforms 1,3-diphenylpro-
purity argon.[10a] Thus,
a solution that behaves as
[a] Department of Chemistry, Box 1652, Southern Illinois Univer-
sity Edwardsville,
Edwardsville, IL 62025-1652, USA
Fax: +1-618-650-3556
E-mail: michsha@siue.edu
[b] Department of Chemistry and Biochemistry,
315 Benton Hall, One University Blvd., St. Louis, MO 63121,
USA
Supporting information for this article is available on the
WWW under http://www.eurjic.org/ or from the author.
Eur. J. Inorg. Chem. 2007, 3943–3946
© 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
3943

M. J. Shaw, S. W. Bryant, N. Rath
SHORT COMMUNICATION
tions of isolated 1(BArF4)2 with alkynones. Additionally, a
1
CH2Cl2 signal (δ =5.32 ppm) is observed in the H NMR
spectrum of samples of 1(BArF4)2, so this solid is formu-
lated as 1(BArF4)2·CH2Cl2, which is consistent with the mi-
croanalytical data. As yet, X-ray quality crystals of this ma-
terial are not available.
In CDCl3 solution, 1(BArF4)2 appears to change into two
species. As expected, there is no evidence of interaction be-
tween the noncoordinating BArF – anion and cation 12+ in
4
the 19F NMR spectrum. The 1H NMR spectra of 1(BArF
)
4 2
in CDCl3 reveal two broad resonances at 4.53 and 5.16 ppm
attributable to C5H5 protons, which indicates that the bulk
of the ruthenium complex exists in two forms. There are
also broad 1H NMR signals at 7.01 and 10.11 ppm assigned
1
to the ortho hydrogen atoms of the PPh3 ligands. The H
Figure 1. Projection view of complex 2a+ with 50% thermal ellip-
soids. The counterion and H atoms are omitted for clarity.
NMR signal at 10.11 ppm disappears upon addition of al-
kynone with a corresponding increase in the usual Ph re-
gion of the spectrum. We suggest that the 10.11 ppm signal
indicates that some of the ruthenium complex converts to
pynone into the 2-phenyl-2-benzoylvinylidene ligand. Cat- a monometallic species agostically stabilized in solution by
ion 2a+ is close in size to the BArF – anion (Supporting ortho C–H interactions as depicted in Scheme 2. Crystallo-
4
Information, Figure S1), a factor that often aids in the graphically characterized examples of related complexes
crystallization process. The solubility of the BArF – salts in with and without agostic interactions are known.[15] It is
4
–
CDCl3 and Et2O is higher than the corresponding PF6
unlikely that this second cation exists as a true 16-electron
salts, which tend to form oily masses rather than X-ray species, as such compounds are known to have intense blue
quality crystals.
color.[14]
Complexes 2a+–2c+ were spectroscopically characterized
by 1H, 13C, 31P, and 19F NMR spectroscopy, IR spec-
troscopy, and ESI-MS as their BArF – salts. For example,
4
the NMR spectra of 2aBArF show the expected features
4
such as a single resonance due to the C5H5 group in the 1H
and 13C NMR spectra, and a single resonance due to the
two equivalent phosphane ligands and the BArF – anion,
4
respectively, in the 31P and 19F NMR spectra. Of note is
the resonance at δ = 349.53 ppm in the 13C NMR spectrum,
which is due to the vinylidene carbon atom attached di-
rectly to the Ru atom.[13] Similarly, terminal vinylidenes
2bBArF and 2cBArF show diagnostic peaks in the 13C
4
4
NMR spectra at 351.91 and 353.29 ppm, respectively (see
Supporting Information). For 2aBArF4, a feature in the IR
spectrum at 1658 cm–1 is attributed to the vinylidene
stretching vibration,[12b] but this vibration also appears to
couple with the ketone stretching frequency that should oc-
Scheme 2. The reaction of 1,3-diphenylpropynone with 1BArF4 and
in-situ-generated 1PF6.
cur in a similar position. The ESI-MS spectrum of 2aBArF
The reaction conditions play a considerable role in this
4
reveals the P+ peak at 897 m/z with the expected pattern of alkyne-to-vinylidene conversion. Solutions of 1(BArF
)
4 2
Ru-isotope peaks.
under an atmosphere of argon react immediately with 1,3-
The nature of the “[CpRu(PPh3)2]+” cation in these reac- diphenylpropynone to form 2aBArF
.
Solutions of
4
tions is an important factor. Solid samples of 1(BArF4)2 can “CpRu(PPh3)2PF6” generated in situ under an atmosphere
be prepared by treating [CpRu(PPh3)2Cl] with NaBArF in of N2 react over several hours to yield a mixture of prod-
4
CH2Cl2 under an argon atmosphere, followed by filtration ucts, in which 2aPF6 can be observed spectroscopically by
1
of the mixture to remove NaCl and recrystallization (see IR and H NMR spectroscopy. Under these conditions an
Supporting Information). The Raman spectra of solid sam- additional isomer of 2aPF6 can be isolated (Scheme 2). The
ples of 1(BArF4)2 show a peak at 2062 cm–1, which is due η1-ketone complex 3aPF6 can be isolated as a red-brown
to the bridging N2 ligand.[10a,14] Exposure of this material powder by fractional crystallization at –30 °C. Compound
in solution to nitrogen gas results in a band at 2192 cm–1 in 3aPF6 was characterized by NMR and IR spectroscopic
the KBr-pellet IR spectrum of the resulting solid as a result methods and ESI-MS. These data establish that cation 3a+
of the formation of a terminal N2 ligand.[10a,14] However, is an isomer of 2a+. The definitive evidence for the η1-
exposure to excess N2 appears to have no effect on the reac- ketone structure of 3aPF6 comes from its IR spectrum
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Eur. J. Inorg. Chem. 2007, 3943–3946

η1-Vinylidene Formation from Alkynones by C–C Bond Migration
SHORT COMMUNICATION
where an alkyne band at 2202 cm–1 is almost unchanged tense theoretical and applied interest, and it may represent
from that of 1,3-diphenylpropynone (2207 cm–1), whereas a general method for the activation of internal alkynes
the ketone band moves to 1524 cm–1, a shift of 116 cm–1 found in acetylenic ketones. Further theoretical and experi-
lower than that of the free ligand.[16] The 13C NMR spectra mental studies are underway to establish a pathway for the
support this assignment with alkyne C atoms observable in activation of the acetylenic ketone ligand.
their usual range, and the carbonyl C atom resonates at
almost the same chemical shift as in the free ligand.[16]
It thus appears that the presence of ligands that coordi-
Experimental Section
–
nate more strongly than BArF – (such as PF6 ) is a factor
4
All manipulations were carried out under an atmosphere of pre-
purified nitrogen or argon. All solvents were distilled from appro-
priate drying agents (e.g. CaH2 or K) and freeze–pump–thaw de-
gassed before use.
that can slow the formation of the vinylidene cation.
CH2Cl2 solutions of “[CpRu(PPh3)2]PF6” generated in situ
by halide abstraction and then exposed to N2 slowly de-
velop IR bands at 2181 cm–1, which are due to terminal N2
Preparation of 2aBArF4: Under an atmosphere of dry argon,
[CpRu(PPh3)2Cl] (1.40 g, 1.93 mmol) was mixed with AgPF6
(0.487 g, 1.78 mmol) in CH2Cl2 (50 mL). The solution was stirred
for 30 min and then 1,3-diphenylpropynone (0.45 g, 2.18 mmol)
was added. The mixture was stirred in the dark for 5 d whereupon
it was filtered, concentrated, and cooled to –30 °C. After 2 d, white
crystals of AgCl had formed, and the solution was transferred to
another vessel, further concentrated, and cooled again to –30 °C.
This procedure was repeated until no more white precipitate
formed, and then a crude solid (1.30 g) was precipitated from the
ligands, as previously observed for similar Ru species.[14]
The addition of 1,3-diphenylpropynone to such solutions
(Scheme 2) immediately induces a change in the burgundy
of 3aPF6. Over a period of hours, the color reverts to
orange-brown. Progress monitored by FTIR spectroscopy
indicates the immediate conversion of some “[CpRu(PPh3)2]-
PF6” into 3aPF6, as evidenced by the appearance of a
strong IR band at 1532 cm–1. However, the alkyne band
and the bound ketone band are very slowly consumed,
whereas features due to vinylidene complex 2aPF6 increase solution by the addition of Et2O (50 mL). Half of the crude solid
(0.62 g) was dissolved in CH2Cl2 (2.0 mL) to which NaBArF
(0.61 g, 0.69 mmol) was added. The solution was stirred for 30 min,
in intensity (see Supporting Information). No features due
4
to an η2-alkyne complex are observed in the region 1800–
2000 cm–1. From these results is it clear that the ketone
filtered, and Et2O (10 mL) was added. The solution was concen-
trated under reduced pressure and cooled to –30 °C overnight
complex 3aPF6 slowly isomerizes in CH2Cl2 to form the
whereupon 2aBArF precipitated from the solution as bright-
4
vinylidene complex 2aPF6.
orange crystals that were suitable for x-ray diffraction studies. Fur-
An η2-alkyne intermediate cannot be ruled out at pres-
ther concentration of the mother liquor resulted in four more crops
ent. The conversion of η1-ketone to η2-alkyne complexes
was previously observed and is facile.[17] Calculations on
[Cp(PMe3)2(η2-HCϵCR)]+ (R = H, CH3) complexes show
of 2aBArF {0.521 g total, estimate 15% yield based on original
4
[CpRu(PPh3)2Cl]}. 1H NMR (300 MHz, CDCl3, 25 °C): δ = 5.12 (s,
C5H5), 6.80–7.78 (m, aromatic) ppm. 13C NMR (75 MHz, CDCl3,
that initial slippage of the Ru atom from the alkyne triple 25 °C): δ = 349.53 (s, Ru=C=C), 188.57 (s, C=O), 95.29 (s, C5H5),
82.00 (s, Ru=C=C) ppm. 31P NMR (121.46 MHz, CDCl3, 25 °C):
bond to the terminal C–H bond is a necessary step in the
isomerization process.[4d] It is likely that activation of the
C(O)R group occurs in these reactions by a similar interac-
tion of the Ru atom with the C–C bond that links the al-
δ = 40.48 (s) ppm. IR (KBr): ν = 1657, 1557, 1439, 1360, 1281,
˜
1128 cm–1. ESI-MS: m/z = 987 [P – BArF –]+. [C88H57BF24OP2Ru]
4
(1760.21): calcd. C 60.05, H 3.26; found C 60.07, H 3.28.
kyne to the C=O unit. Ph group migration is unlikely be- Preparation of 3aPF6: [CpRuCl(PPh3)2] (0.38 g, 0.50 mmol),
cause the addition of diphenylacetylene to 1BArF4 yields no
CH2Cl2 (20 mL), and AgPF6 (0.13 g, 0.50 mmol) were placed in a
100-mL Schlenk tube. The mixture was stirred under inert condi-
vinylidene (monitored by NMR spectroscopy) and at least
one related η2-PhCϵCPh complex was crystallographically
tions, and a voluminous white precipitate formed. The mixture was
stirred for 30 min and 1,3-diphenylpropynone (0.11 g, 0.50 mmol)
characterized.[18] In the formation of the cyclopropyl com-
was then added. The mixture was stirred overnight in the dark and
plex 2cBArF4, no sign of ring-opening occurs during isom-
then filtered through a 1-cm layer of Celite supported on a medium
erization. If the cyclopropyl ring migrates, then migration
porous glass frit. Toluene (2 mL) was then added to induce
must occur without a significant amount of positive charge
crystallization, and the mixture was concentrated under reduced
or unpaired electron density build up on the migrating
group, as these factors would encourage ring opening. De-
pressure. The solution was cooled overnight to –30 °C, and a small
amount of solid was isolated. Three small crops of solid were ob-
tailed calculations on this reaction pathway are currently tained first, all between ca. 2 and 5 mg, which corresponded to
impure AgCl and 1PF6. After these crops were obtained, the solu-
tion began to form a separate layer of oil. The oil was concentrated
under reduced pressure and Et2O was added (10 mL), and the solu-
tion was stirred overnight. The resulting powder of 3PF6 (0.22 g,
42%) was isolated by filtration, rinsed with Et2O, and dried under
vacuum. 1H NMR (300 MHz, CDCl3): δ = 6.9–7.6 (40 H, Ph), 5.22
(5 H, C5H5) ppm. 13C NMR (75 MHz, CDCl3): δ = 189.0 (C=O),
underway.
Conclusions
This report describes the reaction of an internal alkyne
with a commonly encountered d6-Ru complex cation that
results in C–C bond migration to form a vinylidene com-
plex. This migration represents a new path to disubstituted
98 (C H ) ppm. IR (CH Cl ): ν = 2207 [ν(CϵC)], 1524 [ν(C=O)]
˜
5
5
2
2
cm–1. FAB-MS: m/z = 897.1 [P]+, 690.9 [P – dpp]+, 634.8 [P –
PPh3]+. C56H45OF6P3Ru (1041.95): C 64.55, H 4.35; found C 64.44,
terminal vinylidene complexes, which are currently of in- H 4.27.
Eur. J. Inorg. Chem. 2007, 3943–3946
© 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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3945

M. J. Shaw, S. W. Bryant, N. Rath
SHORT COMMUNICATION
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IR Spectroscopic Monitoring of the Reaction of [CpRu(PPh3)2]PF6
with 1,3-Diphenylpropynone: Under an atmosphere of dry argon,
[CpRu(PPh3)2Cl] (0.40 g, 0.96 mmol) was mixed with AgPF6
(0.24 g, 0.96 mmol) in CH2Cl2 (50 mL). The solution was stirred
for 30 min, and a portion (7.0 mL) of the mixture was then trans-
ferred by syringe into a cell equipped with a fiber optic FTIR
probe. Two spectra were recorded that showed a flat baseline and
a peak at 2181 cm–1 [ν(NϵN)] and then 1,3-diphenylpropynone
(0.14 g, 0.67 mmol) was added. Thirty spectra were recorded over
20 min: the spectra contained peaks at 2197 and 1532 cm–1 that
decreased in intensity and at 1658, 1596, 1580, and 1558 cm–1 that
increased in intensity; a strong peak at 1641 cm–1 decreased slightly
in intensity; and an isosbestic point at 1528 cm–1. These spectra are
plotted in Figure S2 (Supporting Information). A KBr-pellet IR
spectrum of 2BArF is plotted in Figure S3.
4
[10] For examples, see: a) H. Aneetha, M. Jimenez-Tenorio, M. C.
Puerta, P. Valerga, K. Mereiter, Organometallics 2002, 21, 628–
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Supporting Information (see footnote on the first page of this arti-
cle): Experimental procedures and characterization data for the
three vinylidene cations 2BArF4, crystal structure of 2BArF4, and
IR spectra.
to –30 °C. Data for 2aBArF at 100 K: C88H57BF24OP2Ru
Acknowledgments
4
–1
¯
1760.16 gmol , triclinic, space group P1, a = 12.3691(8) Å,
b = 17.6970(10) Å, c = 18.1997(11) Å, α = 104.163(3)°, β =
This work was supported by an award to M. J. S. from the United
States National Science Foundation (CHE-0514745) and by the
Southern Illinois University Edwardsville (SIUE) Graduate School.
The X-ray diffractometer was purchased with a grant from NSF
(CHE0420497). We are grateful for the assistance of the Richter-
Addo group at the University of Oklahoma for assistance in ob-
taining ESI-MS.
92.766(4)°, γ = 99.179(3)°, V = 3797.4(4) Å3, Z = 2, Dcalcd.
=
1.539 Mgm–3, R1 = 0.0656 for 12197 reflections with IϾ2σ(I),
wR2 = 0.1199 for 17959 reflections (Rint = 0.074, with a mea-
sured total number of 64083 reflections), GOF on F2 = 1.024,
largest diff. peak (hole) = 1.046 (–0.830) eÅ–3. CCDC-647871
contains the supplementary crystallographic data for this pa-
per. These data can be obtained free of charge from The Cam-
bridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/
data_request/cif.
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Received: June 6, 2007
Published Online: July 27, 2007
3946
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Eur. J. Inorg. Chem. 2007, 3943–3946